Charles Krug says...

I hit Calc as a HS Senior in 1980, then again as a college freshman.

MUCH later when I finished, I did Advanced Calculus (Sometimes called
"Introductory Real Analysis" where you do all the proving.

The other "prove it" courses were Discrete Mathematics (all about
counting) and "Modern Algebra" (Properties of sets, operations, groups,
rings, fields . . . )

Great fun.

A math major? Mine was chemistry. I did differential and integral
calculus, analytic geometry and multivariate calculus with relative
ease. Then I got to linear algebra and differential equations and my
brain stopped working. It didn't help that we had a fresh PhD whiz kid
as the prof who hadn't learned how to dumb it down yet to us poor slobs
who were only minoring in math, not making it a career. Got a B in the
class, but it was only because everybody was flunking and he had to
resort to the curve to end all curves so that everybody didn't get an F.
My first semester calc instructor told us on day one there would be no
curve, even if it meant failing everybody. At the end, he said we were
the best calculus class he ever had and that four people had earned A's
(including me) and he hadn't given any A's at all in the previous three
semesters. The class was an hour long and he gave three hour tests--one
every two weeks and a take home test to go with the in-class test. We
took our final exam in the library because it was open until 10:00PM.
Our class time was at 6:00PM and he said he would be in the library at
4:30 if anyone wanted to start the test then. I arrived at 4:30 and
turned in my exam when the library was closing. Out of 25 story
problems, I still left three blank after 5 1/2 hours of work.